Synthesis of ribosomal RNA (rRNA) is catalyzed by DNA-dependent RNA polymerase I (Pol I). Pol I levels and properties have been compared in liver and hepatomas. Hepatomas contain a form of the enzyme which is distinct from that of normal liver. Two classes of hepatoma were observed. One class has 5-10 times more enzyme than normal liver while the second has levels similar to those of liver. Both classes of tumor synthesize rRNA at the same rate in culture. The enzyme in a high Pol I strain is structurally altered, as determined by thermal denaturation studies.